Donepezil hydrochloride (I) has excellent action as a prophylactic and a therapeutic agent for senile dementia, and in particular as a prophylactic and therapeutic agent for Alzheimer's disease and an industrial process for producing the same, has been reported.
The process for the preparation of 1-benzyl-4-[(5,6-dimethoxy-1-indanone)-2-yl] methyl piperidine has been described in JP A-64-79151 (U.S. Pat. No. 4,895,841, EP 296560).

Japanese patent application, publication No. A-100-53576 (WO 9746527) discloses certain forms (I, II, III, IV & V) of Donepezil hydrochloride, 1-benzyl-4-[(5,6-dimethoxy-1-indanone)-2-yl] methyl piperidine hydrochloride. This patent also discloses an amorphous form of Donepezil hydrochloride, which is reported to be chemically unstable on storage.
U.S. Pat. No. 6,734,195 claims a chemically stable amorphous form of Donepezil hydrochloride and its use in formulation.
The U.S. Pat. No. 6,734,195 however has not reported any polymorphic stability of amorphous form. However the above JP patent A-100-53576 (WO 9746527) reports conversion of amorphous form to crystalline form (IV) when exposed to more than 90% relative humidity at room temperature.